Modulating system



j Sept. 12, 1933.

J. c. SCHELLENG 1,926,886

MODULATING .SYSTEM Filed June 22, 1929 ATTO/PNE) Patented Sept. 12, 1933 MODULATING SYSTEM John C. Schelleng, Millburn, N. J., assignor to Bell Telephone Laboratories, Incorporated, 'New'York, N. Y., a corporation of New York Application June 22, 1929. Serial No. 372,840

4 Claims. (o1.-179 171) This invention relates to modulation systems and more'particularly to modulation systems in which the carrier and one side band'are sup? pressed.

It is an object of the present invention to so modulate a high frequency wave with a low frequency wave that only one side band of the modulation products is produced in the output circuit of the modulator and is available for transmission'to a load circuit.

Another objectof this invention is to modulate a high frequency wave in accordance with a low frequency wave in such a; manner that the output energysupplied to the load circuit is a maximum.

Another object is to effect modulation in balanced space discharge devices so that no -singing occurs. g

In, one embodiment of the invention the carrier frequency wave is applied through a parallel connection to the grids of balanced space discharge devices and'the low'frequency wave, after being amplified by space discharge devices connected in balanced or push-pull relation, is applied to modulate'the carrier wave in the space paths of the carrier wave discharge devices.

By virtue of the method of applyingthe carrier wave to the common input circuit of the discharge devices components of this frequency produced in their common output'circuit will neutralize eachother and hence no carrier frequency energy will be supplied to the load circuit. The low frequency devices and the high frequency devices' are' supplied'with space current from a common source in accordance with the constant. current method of modulation disclosed in the UL, S. Patent 1,442,147,. January 16, 1923, to Heising. 'The amplified low frequency wave is impressed upon the plates of the carrier frequency devices. to effect modulation of the carrier waveg In'order to secure maximum output fromja modulating system of the typedescribed above the impedance of the output circuit for the carrier wave-should be zero and, for the modulation products to be. supplied to the load circuit, the output impedance should be of the same order as the internal outputimpedance of the modulator.

In accordance with the present invention, the combined output circuit of the devices in which modulation occurs contains a circuit which is parallel resonant to one side band component ofthe modulated'wave and alsoincludesa path, series resonant for the carrier frequency which isconnected between theplate of each discharge device and ground.

The parallel resonant circuit operates to select the side band to be transmitted to the load circuit and to suppress the other side band. It is also designed to match the internal impedance of the discharge devices.

The series resonant circuit, being of zero impedance to the carrier wave, precludes the production of carrier frequency voltage in the plate circuit andv thereby prevents parallel singing.

The grids of the discharge devices, constituting the modulator, are connected together by condensers of such value that they virtually act as a short circuit for energy of the carrier fre- 70..

quency, whereby push-pull fsing'mg is prevented.

Other features of the invention will be apparent from thefollowing description in connection with the accompanying drawing, Fig. 1 of which illustrates schematically one embodiment of the 75,

invention while Fig. 2 illustrates'theadaptation ofv the'arrang'ement of Fig. 1 to a system whereby several messages may be transmitted on one carrier. q

Referring to Fig. 1' a pair of vacuum tubes 5 and. 6 are so interconnected with each other and with a source of carrier frequency oscillations 3, through a tuned circuit 4, that the oscillations impressed on the gridsof the tubes are always of the same polarity and are'of equal amplitude.

A grid biasing'arrangeme'nt,consisting of potentiometers 7 and 8 and a voltage source 9, facilitate the proper biasingof the grids and, by virtue of the adjustabilit'y of '7 and 8, permit the tube circuits to'be accurately balanced.v CapacitieslO and 11. are of such value that they present practically no impedance .to oscillations of the frequency'of source 3'. The grids of tubes 5 and 6; being'connected by a virtual'short circuit, no voltage .can buildup 'therebetween with the result that singing? over a series path in the tube circuit cannot occur. 4

The filaments of tubes 5 and 6 are heated by any convenient source; represented as battery 12 and are grounded at some point 13.

The plates of the, tubes have impressed'upon them a voltage from source 14, indicated as a battery, over an obvious circuit.

The. alternating current output circuit of the tubes 5 and Scomprises coils 15 and 16 and capacities 1'7 and 18, in parallel with capacity 19. This circuit is grounded through variable capacity 20.

A source of signals, represented by block 21; is coupled by a transformer 22 to' the grids'of vacuum tubes 23 and 24. Capacity 25 and the secondary winding of transformer 22 constitutes a circuit tuned to the frequency of the applied signal currents.

As is well known, the impressed signal current causes the grids of tubes 23 and 24 to be oppositely poled at any given instant. This method of applying energy to a balanced circuit, including opposed cooperating devices, is sometimes spoken of as push-pull application.

When the band of frequencies from source 21 is wide relative to the mean frequency, as in speech, the input and output circuits of tubes 3 and 24 would have to be broadly tuned to such band instead of sharply tuned to a single frequency as in the case of single tone telegraphy.

Capacities 26 and 27 connected from the grid of one tube to the plate of the other tube constitute a well known method of neutralizing the grid to plate capacities of vacuum tubes 23 and 24 constituting the amplifier.

The plates of tubes 23 and 24 have impressed upon them a voltage from the common source 14. The common alternating current output circuit for the tubes23 and 24, comprises inductance 28 and variable capacity 29.

Source 14 is connected to the plates of tubes 5 and 6 and tubes 23 and 24 through a constant current choke coil 30. Radio frequency choke coils 31 and 32 are inserted in the plate leads of tubes 5 and 6 to keep radio frequency oscillations out of the circuits of the tubes 23 and 24.

As described in the above mentioned Heising patent the carrier frequency oscillations, re-

. peated by the tubes 5 and 6, have their ampli- 35" tudes Varied in accordance with the signal waves amplified by tubes 23 and 24. Tubes 23 and 24 are low or signal frequency amplifiers arranged in push-pull relation, and tubes 5 and 6 constitute a balanced modulator.

The modulated currents are led'off to an antenna or to high power amplifier tubes through leads 33 and 34 by means of coils 35 and 36 which are inductively coupled to coils 15 and 16.

The common output circuit of tubes 5 and 6 include twopaths, one comprising coil 15, capacity 1'? and variable capacity 20 and the other comprising coil 16, capacity 18 and variable capacity 20, which jointly constitute a series circuit resonant to the carrier frequency supplied by source 3. The output path for the carrier frequency oscillations is from ground 13, through the filament and plate of each tube 5 and 6, coil 15, capacity 17 for one tube and coil 16 and capacity 18 for the other tube and thevariable capacity 20 to ground. The currents in the two branches are equal and, asfar as their effect in coils 35 and 36 is concerned, are opposite. Hence the unmodulated, carrier frequency component is not supplied to the circuit 3334.

Also since the circuit traced above from the plates of tubes 5 and 6 to ground is series resonant to the carrier or high frequency currents, the impedance to such currents in each circuit is practically zero and hence no voltage having a frequency of source 3 can build up to cause parallel singing. Furthermore, singing of the parallel type would require the impedances of grid and plate circuits separately to be inductive at the singing frequency. However, the grid circuit being parallel tuned and the plate circuit being series tuned to the same frequency the reactance components of these impedances are always opposite so that for any frequency one of them is capacitive.

The parallel circuit comprising variable capacity l9, coils 15 and 16 and capacities 17 and 18 may be tuned to the side band which it is desired to transmit and at the same time have the optimum impedance relative to that of the internal impedance of the tubes. The undesired side band is therefore suppressed and the output energy of the desired side band is a maximum.

With the circuits just described and with source 3 generating oscillationsof a frequency suitable for radio transmission, considerable difficulty would be encountered were an attempt made to provide the common output circuit of the tubes 5 and 6 with a tuned circuit adapted to select one of the side band components While suppressing the other side band component and any carrier component which may appear in the output circuit of the tubes, due to lack of symmetry of the circuit.

To illustrate, let it be assumed that source 21 supplies signal currents of 1,500 cycles and that source 3 supplies oscillations of 1,000,000 cycles. The resulting side bands would be 1,001,500 cycles and 998,500 cycles. In such a case, it Would'in practice be prohibitive to provide a tuned circuit which could select one of the side bands while suppressing the other.

However, the arrangement described above may be utilized in each stage of a plural modulation system, in which the carrier frequency is increased with succeeding stages. The first stage may then be designed to operate with signal currents within the voice frequency range and a carrier of the order of 10,000 cycles. Under these conditions, it would be possibile to provide selective means for simultaneously selecting one side band and suppressing the other side band and the unmodulated carrier component. In the succeeding stages the carrier may be greatly increased in frequency. This invention contemplates such a plural modulation system, each modulation stage of which may be of the type described above.

Fig. 2 illustrates a system, similar to that're ferred to in the previous paragraph. This figure shows a system adapted for the transmission of a plurality of messages as modulations of asingle radio frequency wave.

A plurality of single side band modulators, having the general characteristics of Fig. 1, are represented at 40,41 and 42. By way of example, the low frequency carrier supplied to the modulator 40 may be 60,000 cycles, that supplied to modulator 41 may be 70,000 cycles, and that supplied to modulator 42 may be 80,000 cycles. The side bands transmitted preferably bear the same relation to their carriers, that is, they are either all upper or all lower side bands.

Filter 43, 44 and 45 are inserted in the output leads of the respective modulators to prevent interaction of the different currents upon the other modulators.

The currents from the filters are amplified by a vacuum tube system 46, which maybe similar to that including tubes 23 and 24 of'Fig. 1.,

A source of radio frequency oscillations, which may be controlled by a piezoelectric crystal, is

represented by block 47.

The output currents from amplifier 46 and oscillations from source 47 are impressed upon a modulator 48 which is preferably of the type illustrated similar to that including tubes 5 and 6 of Fig. 1.

The output circuit of modulator 48 is adapted to suppress the unmodulated carrier component foo fro

amplifiers before being supplied to the transmission circuit.

To illustrate the operation of the system of Fig. 2, assume that tones of 1,500 cycles, 2,000 cycles and 2,500 cycles are used to respectively modulate carrier waves of 60,000 cycles, 70,000

' cycles and 80,000 cycles supplies to the modulators 40, 41 and 42, respectively, and also assume that the upper side band is selected in each case. Amplifier 46 will, then have impressed upon it currents having frequencies of 61,500 cycles,'72,000 cycles and 82,500 cycles.

Also assume that oscillator 47 is generating a wave having a frequency ,of 1,000,000 cycles. After modulation in one case the side bands will be 1,061,500 and 938,500, in the second case 1,072,000 and 928,000 and inthe third case 1,082,500 and 917,500.

As will be readily apparent, all of the upper side bands may be selected and the lower side bands suppressed, since a spacing of, 123,000 cycles exists between the frequencies of the nearest side bands produced, and circuits may be designed to pass a band from 1,061,500 cycles to 1,082,500 cycles and suppress frequencies more than 100,000 cycles below the lower limit.

It is to be understoodthat modifications may be made in the circuits described herein, without departing from the spirit and scope of this invention. Also, while numerical values have been given for illustrative purposes, the invention is not to be limited thereto, as the principles of the invention are not dependent upon the figures herein given.

In order to provide for selecting one side band and suppressing the other side band and unmodulated component, the modulators 40, 41 and 42 may, for example, be supplied with single side bands from a modulator, or modulators, constituting a source 21, in which a low frequency carrier wave is modulated by signal currents, as described above.

What is claimed is:

1. A system comprising a space discharge device having a cathode, an anode and a grid, means for impressing a carrier wave upon the grid of saiddevice, means for supplying modulating current to the device, an output circuit comprising a path from the anode to the cathode series reso-.

nant at the carrier frequency, and an impedance shunting a portion of said path whereby the output circuit is rendered anti-resonant at one sideband frequency.

2. A system according to the preceding claim in which the shunting impedance includes a con-' denser.

3. A system comprising a pair of three-electrode space discharge devices connected inpush-pull arrangement, a common path from the cathodes of said devices to their grids including a source of carriers waves, means for supplying modulating current to said devices, a path between the anodes of said devices anti-resonant to a side-band frequency, and a path-from each anode to the cathodes including atleast a portion of said anti-resonant path and series resonant at the carrier frequency.

4, A system comprising a pair .of three-electrode space discharge devices connected in pushpull arrangement, a common path from the oathodes to the grids of said devices including a source of carrier waves, and including a .connection between said grids of substantially zero modulating current to the devices, a pathbetween the anodes of said devices anti-resonant to a sideband frequency, and a path from each anode to the cathodes including at least a portion or" said anti-resonant path and series resonant to the carrier.

' J OHN C. SCHELLENG.

impedance to the carrier, means for supplying" 

